Random bred, male Fisher 344 rats, CD-1 mice, and golden Syrian hamsters were exposed to 1-2 ppm of NO2 for 6 hr/da, 5 da/wk for 30 weeks. Some of the animals also received 0.1% morpholine in deionized water as their sole source of drinking water. The animals will be held for 2 years or until death during which time all animals will receive deionized water. Animals will be killed for histopathological evaluation following the postexposure holding period.